Understanding the Encapsulation Behavior of Nile Red within Nonionic Block Copolymer Coacervates and Self-Assembled Particles

The encapsulation of small molecules within nonionic block copolymer coacervates holds significant potential across many fields. However, the mechanism driving this process remains largely unexplored. Structural characterization by cryogenic electron microscopy reveals sponge-like microstructures with compartmentalized domains, suggesting a physical basis for selective partitioning. Here, we investigate the partitioning behavior of the hydrophobic dye Nile red in two nonionic block copolymer systems: polyethylene glycol-block-poly(methyl methacrylate) (PEG45-b-PMMA) and polyethylene glycol-block-polycaprolactone (PEG45-b-PCL). We employ confocal laser scanning microscopy coupled with automated image analysis to quantify partition coefficients as a function of degree of polymerization (DP) of the PMMA block, dye concentration, and water. Our results show that the partition coefficient increases with PMMA block and water. These results show that the relative solubility between the dilute and the dense phases plays a dominant role in partitioning.